Methods for inspecting an electrically conductive object by means of eddy current measurements, in particular a transient eddy current measurement, are known in the art. These are also referred to as pulsed eddy current (PEC) methods. For example, to estimate wall thickness of an electrically conductive object, a pulsed eddy current probe comprising a transmitter/receiver arrangement with a transmitter means and a receiver means is placed in proximity with the near surface of the inspection object. The transmitter means, typically comprising an electromagnetic coil or coil arrangement, is activated, e.g. by energizing and abruptly de-energizing the coil, and this induces transient eddy currents in the object. The eddy currents give rise to a time-varying magnetic field, which generates a signal in the receiver means, typically also comprising an electromagnetic coil or coil arrangement. Often, the received signal is measured as a function of time after the transmitter pulse. Analysis of the signal yields information about the inspection object, e.g. wall thickness variations can be measured, anomalies, cracks or surface defects can be located. Examples of prior art methods are discussed in patent publications EP 321 112 A1, EP 1311800 A1, EP 1311842 A1, EP 1311843 A1, EP 1478899 A2, EP 1576335 A2.
U.S. Pat. No. 2,534,420 relates to a method and apparatus for testing a limited area of a non-magnetic material particularly for its thickness, composition, properties, heat-treated condition, structure, and presence of defects. The method comprises placing the material to be tested between two magnets each having an inner pole surrounded by an outer pole. The poles are arranged so that when an AC source is applied to the magnets, a magnetic flux passes through the material in one direction between the inner poles and returns through the material between the outer poles in the opposite direction. This generates a flow of eddy currents in the material that is limited to the area enclosed by the outer poles.
GB 2,262,346 relates to a method and apparatus for detecting surface defects, such as pitting corrosion, existing on the surface of steel materials based on the application of eddy currents. The method comprises generating an eddy current by supplying an alternating current to a primary coil in an eddy current sensor.
U.S. Pat. No. 6,636,037 relates to devices and methods for inspections of conducting objects, such as flat shaped conducting structures, as well as conducting structures having different shapes. The method comprises transmitting an alternating magnetic signal to the conducting object.
GB 2,329,024 D5 relates to a pipeline inspection vehicle which can be towed through a pipeline which comprises a means for generating an eddy current field in the pipeline and means for detecting the resultant field following passage through the structure. The eddy current is induced by an alternating current coil.
A problem is encountered in eddy current measurements when the object to be inspected is located in the vicinity of another electrically conductive object, since the other object has an influence on the measurements of the inspection object. This is for example the case when one of two concentric pipes is to be inspected for corrosion from the annulus between those pipes. Such a task is for example encountered in hydrocarbon wells which are completed with concentric steel tubulars, referred to as casing, conductors, or tubing. The only practical access in such inspection is via an annulus of typically 100 mm width. Because this spacing is so small, the presence of the adjacent tubular makes it difficult to perform eddy current inspection on a desired subject tubular.
There are many other instances in which eddy current inspection may be precluded by the proximity of a second conducting object.
Thus, there is a need for a device and method that allows eddy current inspection of an inspection object in the vicinity of a further object.